Microwave enhanced CVD method for depositing carbon

ABSTRACT

A method of making a diamond or diamond-like carbon containing 0.01-1.0 weight percent nitrogen on a substrate where an electromagnetic energy is applied to a productive gas comprising carbon and an additive gas comprising nitrogen and the above carbon product is deposited on a substrate.

BACKGROUND OF THE INVENTION

This invention relates to a microwave enhanced CVD method for Depositingcarbon.

Recently, ECR CVD has attracted the interests of researchers as a newmethod of manufacturing thin films, particularly amorphous thin films.For example, Matsuo et al discloses one type of such an ECR CVDapparatus in USP 4,401,054. This recent technique utilizes microwaves toenergize a reactive gas into a plasma state by virtue of a magneticfield which functions to pinch the plasma gas within the excitationspace. With this configuration, the reactive gas can absorb the energyof the microwaves. A substrate to be coated is located distant from theexcitation space (resonating space) for preventing the same from beingspattered. The energized gas is showered on the substrate from theresonating space. In order to establish an electron cyclotron resonance,the pressure in a resonating space is kept at 1 × 10⁻³ to 1 × 10⁻⁵ Torrat which electrons can be considered as independent particle andresonate with a microwave in an electron cyclotron resonance on acertain surface on which the magnetic field takes a particular strengthrequired for ECR. The excited plasma is extrated from the resonatingspace, by means of a divergent magnetic field, to a deposition spacewhich is located distant from the resonating space and in which isdisposed a substrate to be coated.

In such a prior art method, it is very difficult to form a thin film ofa polycrystalline or single-crystalline structure, so that currentlyavailable methods are almost limited to processes for manufacturingamourphous films. Also, high energy chemical vapor reaction is difficultto take place in accordance with such a prior art and therefore adiamond film or other films having high melting points, or uniform filmson an even surface having depressions and caves can not be formed.

Furthermore, it was impossible to coat the surface of a super hard metalsuch as tungsten carbide with a carbon film. Because of this, it isrequired to coat a super hard surface with a fine powder of diamond foruse of abrasive which has a sufficient hardness and to make sturdymechanical contact between the diamond powder and the substrate surface.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a microwaveenhanced CVD method for depositing a carbon.

According to one aspect of the invention, in addition to a carboncompound, nitrogen and/or a nitrogen compound gas is inputted to thereaction chamber. The inputted nitrogen functions to prevent latticedefects from growing by virtue of external or internal stress. When aboron compound is also inputted together with the nitrogen compound, theadhesivity of carbon deposited is improved. Boron nitride appears to bethe binder between the carbon and the underlying substrate to be coatedsuch as made of super hard metal, for example tungsten carbide.Preferably, carbon and boron nitride are deposited on the substrate inthe form of crystalline grain particles or a layer containing nitrogenand boron at less than 10%.

According to another aspect of the invention, a new CVD process has beenculminated. The new process utilizes a mixed cyclotron resonance whichwas introduced firstly by the inventors. In the new type of excitingprocess, a sonic action of reactive gas itself must be taken intoconsideration as a non-negligible perturbation besides the interactionbetween respective particles of the reactive gas and magnetic field andmicrowave, and therefore charged particles of a reactive gas can beabsorbed in a relatively wide resonating space. Preferably, the pressureis maintained higher than 3 Torr. For the mixed resonance, the pressurein a reaction chamber is elevated 10² -10⁵ times as high as that ofprior art. For example, the mixed renonance can be established byincreasing the pressure after ECR takes place at a low pressure. Namely,first a plasma gas is placed in ECR condition at 1 × 10⁻³ to 1 × 10⁻⁵Torr by inputting microwave under the existence of magnetic field. Thena reactive gas is inputted into the plasma gas so that the pressure iselevated to 0.1 to 300 Torr and the resonance is changed from ECR to MCR(Mixed Cyclotron Resonance). Carbon can be decomposed and undergo anecessary reaction at only such a comparatively high pressure. Inprocess, diamond is likely to grow selectively on convexies.

Although carbon is deposited also in an amorphous phase when diamond ispreferred, hydrogen in a plasma state eliminates preferentiallyamorphous carbon by etching, remaining crystalline carbon.

It has been found that the hardness of the diamond formed by the presentinvention is 1.3 to 3.0 times as high as that of diamond which has beenmade by prior art vapor phase method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view showing a CVD apparatus in accordancewith the present invention.

FIG. 2(A) is a graphical diagram showing the profile of theequipotential surfaces of magnetic field in cross section in accordancewith a computer simulation.

FIG. 2(B) is a graphical diagram showing the strength of electric fieldin accordance with a computer simulation.

FIGS. 3(A) and 3(B) are graphical diagrams showing equipotentialsurfaces in terms of magnetic field and electric field of microwavepropagating in a resonating space respectively.

FIG. 4 is a cross sectional view showing another CVD apparatus fordepositing a carbon film in accordance with the present invention byvirtue of a R.F. power.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a microwave enhanced plasma CVD apparatus inaccordance with the present invention is illustrated. In the figure, theapparatus comprises a reaction chamber in which a plasma generatingspace 1 and an auxiliary space 2 are defined and can be maintained at anappropriate negative pressure, a microwave generator 4, electro-magnets5 and 5' in the form of helmholtz coils surrounding the space 1, a powersupply 25 for supplying an electric power to the electro-magnets 5 and5', and a water cooling system 18. The plasma generating space 1 has acircular cross section. In the plasma generating space 1, a substrateholder 10' made of a material which less disturbs the condition ofmagnetic field created by the magnets 5 and 5' in the chamber, e.g.,made of stainless steel or quartz is provided on which a substrate 10 ismounted. The substrate holder 10' is irradiated and heated to 800-1000°C. in the atmosphere of a high temperature plasma gas with an infraredlight 24, which is emitted from an IR heater 20, reflected from an IRreflection parabola mirror 21 and focused on the back surface of theholder 10' through a lens 22. A reference numeral 23 designates a powersupply for the IR heater 20. Provided for evacuating the reactionchamber is an evacuating system comprising a turbo molecular pump 17 anda rotary pump 14 which are connected with the reaction chamber throughpressure controlling valves 11, 13 and 15. The substrate temperature mayreach to a sufficient level only in virtue of the plasma gas generatedin the reaction chamber. In the case, the heater can be dispensed with.Further, depending on the condition of the plasma, the substratetemperature might elevate too high to undergo a suitable reaction. Inthe case, cooling means has to be provided. The process with thisapparatus is carried out as follows.

A substrate 10 is mounted on the substrate holder 10', and the reactionchamber is evacuated to 1 × 10⁻⁶ Torr or a higher vacuum condition.Then, hydrogen gas is introduced from a gas introducing system 6 at 30SCCM, and a microwave of 500 Watt at 2.45GHz is emitted from themicrowave generator 4 through a microwave introduction window 15 to theplasma generating space 1 which is subjected to an magnetic field ofabout 2 K Gauss induced by the magnets 5 and 5'. The hydrogen is excitedinto a high density plasma state in the space 1 at 1 × 10⁻⁴ Torr by theenergy of the microwave. The surface of the substrate is cleaned by highenergy electrons and hydrogen atoms. In addition to the introduction ofthe hydrogen gas, a carbon compound gas as the productive gas such as C₂H₂, C₂ H₄, CH₃ OH, C₂ H₅ OH or CH₄ are inputted at 30 SCCM through anintroduction system 7. In this process the productive gas is dilutedwith hydrogen at a sufficiently thin density, e.g., 0.1 to 5%. Furtherin addition to this, a nitrogen or its compound gas, such as ammonia ornitrogen gas, is inputted to the reaction chamber from the introductionsystem. The proportion of the nitrogen compound gas to the carboncompound gas is 0.1%-5%. Then, the pressure in the reaction chamber ismaintained at 0.1 Torr-300 Torr, preferably 3-30 Torr, e.g., 1 Torr. Byincreasing this pressure in the reaction chamber, it is possible to makehigh the density of the productive gas and, therefore, faster the growthrate of the product. Namely, carbon atoms are excited in a high energycondition so that the substrate 10 mounted on the holder 10' is coatedwith carbon in the form of a film made of i-carbon (insulated carbonconsisting of microcrystals) or diamond having 0.1 to 100 microns ingrain diameter. The deposited carbon contains nitrogen at 0.01 - 1weight %.

Experimental polishing using abrasives coated with a diamond includingnitrogen in accordance with the present invention and a prior artdiamond devoid of nitrogen has been made for the purpose of comparison.As a result, the degradation of the former in polishing power was halfor less of the latter. Namely, the diamond according to the inventionhas a high resistance to wear.

Next, another embodiment is described. A substrate 10 is mounted on thesubstrate holder 10', and the reaction chamber is evacuated to 1 × 10⁻⁶Torr or a higher vacuum condition. Then, hydrogen gas is introduced froma gas introducing system 6 at 300 SCCM, and a microwave of 1 Kilo Wattat 2.45GHz is emitted from the microwave generator 4 through a microwaveintroduction window 15 to the plasma generating space 1 which issubjected to an magnetic field of about 2 K Gauss induced by the magnets5 and 5'. The hydrogen is excited into a high density plasma state inthe space 1 by the energy of the microwave. The surface of the substrateis cleaned by high energy electrons and hydrogen atoms. In addition tothe introduction of the hydrogen gas, a carbon compound gas as theproductive gas such as C₂ H₂, C₂ H₄, CH₃ OH, C₂ H₅ OH or CH₄ areinputted at 3 SCCM through an introduction system 7. In this process,the productive gas is diluted with hydrogen at a sufficiently thindensity, e.g., 0.1 to 15%. Further in addition to this, a nitrogencompound gas such as ammonia, NO₂, NO, N₂ or nitrogen gas, and B₂ H₆ orBF₃ are inputted to the reaction chamber from the introduction systems 7and 8 respectively at B/N = 1. The proportion of B₂ H₆ (BF₃)+NH₃ to thecarbon compound gas is 1%-50%. Then, the pressure in the reactionchamber is maintained at 1 Torr-760 Torr, preferably higher than 10 Torror 10-100 Torr, e.g., 30 Torr. By increasing this pressure in thereaction chamber, it is possible to make high the density of theproductive gas and, therefore, faster the growth rate of the product.Namely, the substrate 10 mounted on the holder 10' is coated with carboncontaining nitrogen and boron (or in the form of boron nitride). Theproduct includes carbon and boron nitride as the main components, thesum of whose proportions is at least 90%.

FIG. 2(A) is a graphical diagram showing the distribution of magneticfield on the region 30 in FIG. 1. Curves on the diagram are plottedalong equipotential surfaces and given numerals indicating the strengthson the respective curves of the magnetic field induced by the magnets 5and 5' having a power of 2000 Gauss. By adjusting the power of themagnets 5 and 5', the strength of the magnetic field can be controlledso that the magnetic field becomes largely uniform over the surface tobe coated which is located in the region 100 where the magnetic field(875±185 Gauss) and the electric field interact. In the diagram, areference 26 designates the equipotential surface of 875 Gauss at whichECR (electron cyclotron resonance) condition between the magnetic fieldand the frequency of the microwave is satisfied. Of course, inaccordance with the present invention, ECR can not be established due tothe high pressure in the reaction chamber, but instead a mixed cyclotronresonance (MCR) takes place in a broad region including theequipotential surface of the ECR condition. FIG. 2(B) is a graphicaldiagram of which the X-axis corresponds to that of FIG. 2(A) and showsthe strength of electric field of the microwave in the plasma generatingspace 1. The strength of the electric field takes its maximum value inthe regions 100 and 100'. However, in the region 100', it is difficultto heat the substrate 10' without disturbing the propagation of themicrowave. In other region a film is not uniformly deposited, butdeposited the product in the form of a doughnut. It is for this reasonthat the substrate 10 is disposed in the region 100. The plasma flows inthe lateral direction. According to the experimental, a uniform film canbe formed on a circular substrate having a diameter of up to 100mm.Preferably, a film is formed in the chamber on a circular substratehaving a diameter of up to 50mm with a uniform thickness and a uniformquality. When a larger substrate is desired to be coated, the diameterof the space 1 can be sized double with respect to the verticaldirection of FIG. 2(A) by making use of 1.225 GHz as the frequency ofthe microwave. FIGS. 3(A) and 3(B) are graphical diagrams showing thedistributions of the magnetic field and the electric field due tomicrowave emitted from the microwave generator 4 on a cross section ofthe plasma generating space 1. The curves in the circles of the figuresare plotted along equipotential surfaces and given numerals showing thestrength. As shown in FIG. 3(B), the electric field reaches its maximumvalue at 25 KV/m.

On the electron beam reflection image of the thin film produced inaccordance with the above procedure, observed are spots indicating thepresence of polycrystalline boron nitride and crystal carbon, i.e.,diamond (single-crystalline particles). Namely, the film is made of themixture of boron nitride and diamond. As the microwave power isincreased from 1KW to 5KW, the proportion of diamond in the filmincreases.

When BF₃ and/or NF₃ is used as the boron and/or nitrogen source, theplasma gas becomes containing fluorine and which fluorine functions toeliminate impurity residing on the surface to be coated by etching.

For reference, a film formation process was performed in the same manneras in the above but without using a magnetic field. As a result, agraphite film was deposited.

By a similar process, amorphous or microcrystalline film can also bedeposited by appropriately selecting the deposition condition. Anamorphous film is deposited when carbon compound gas is diluted with thelarger amount of hydrogen gas, when the input power is comparativelysmall and when the process temperature is comparatively low. When DCbias current is superimposed on the alternating current in thedeposition condition suitable for amorphous, the deposited film becomesincluding microcrystalline structure.

It is a significant feature of the invention that the carbon formed inaccordance with the invention has a very high hardness irrespective ofwhether the carbon is amorphous or crystalline. The Vickers hardness is2000-6400 Kg/mm², e.g., 2000 Kg/mm². The thermal conductivity is notlower than 2.5 W/cmdeg, e.g., 5.0-6.6 W/cmdeg.

The present invention can be applied for the formation of carbon bymeans of glow or arc discharge enhanced CVD caused by an r.f. power.FIG. 4 is a cross sectional view showing a CVD apparatus for depositionby virtue of an r.f. power. In the figure, the apparatus comprises areaction chamber 101, a loading chamber 103, a rotary pump 105 forevacuating the loading chamber 103, a turbo molecular pump 107associated with a rotary pump 109 for evacuating both the reactionchamber 101 and the loading chamber 103, a gas feeding system 127 forinputting process gas such as reactive gas or dopant gas through anozzle 129, a substrate holder 111 for supporting substrates 113,electrodes 115 disposed opposite to the holder 111, an RF power supply117 consisting of a radiofrequency power source 119 associated with amatching circuit 121 and a DC bias circuit 123 for supply an r.f. powerbetween the electrodes 115 and the substrate holder 111, and a halogenlamp heater 125 with a quartz window 129 for heating the substrates 113.The deposition process for coating the substrates 113 with a carbon filmis as follow.

After disposing the substrates 113 in the reaction chamber 101 through agate 129, a reactive gas composed of a gaseous carbon compound such asCH₄, C₂ H₄ and C₂ H₂, and a dopant gas such as nitrogen, a nitrogencompound gas and a boron compound gas if necessary were inputted to thereaction chamber at 1 × 10⁻³ to 5 × 10⁻¹ Torr. The carbon compound gaswas diluted with hydrogen at 50 mol%. At the same time, the substrates113 were heated to not higher than 450° C. by means of the heater 125.In this condition, a vapor reaction was initiated by means of r.f. powerinputted from the power supply 117. The r.f. power was 50 W to 1 KW(0.03 to 3.00 W/cm²) at 13.56MHz superimposed on an DC bias voltage of-200V to +400V. Then, carbon films were deposited on the substrates 113at a growth rate of 150 Å/min. The carbon film looked like an amorphousstructure rather than a crystalline structure. Despite the amorphousstructure, the hardness was measured as high as that of a diamond film.The Vickers hardness thereof was 2000-6400 Kg/mm², e.g., 2000 Kg/mm². Sowe call it "diamond-like carbon" or DLC for short.

In accordance with the present invention, a super lattice structure canbe also formed. A boron nitride (BN) thin film is deposited in the sameway as illustrated in the above but without using carbon compound gas. Acarbon thin film and a BN thin film are deposited in turn many times sothat a super lattice structure is stacked on a substrate.

The invention should not limited to the above particular embodiments andmany modifications and variations may cause to those skilled in the art.For example, it has been proved effective to add aluminium orphosphorous into carbon at 0.001 to 1 weight%. Although the reactive gasis let flow from a side to the right, the system can be designed so thatthe gas flows from left to right, or upward or downward.

I claim:
 1. A method of making a diamond or diamond-like carboncontaining 0.01 -1.0 weight percent nitrogen on a substratecomprising:placing said substrate in a reaction chamber; inputting aproductive gas comprising carbon together with an additive gascomprising nitrogen into said reaction chamber; applying anelectromagnetic energy to activate said productive and additive gases;and depositing said carbon product containing nitrogen on a surface ofsaid substrate.
 2. The method of claim 1 wherein, in said applying step,microwave energy and a magnetic field activate the said productive andadditive gases.
 3. The method of claim 2 wherein the frequency of saidmicrowave is 2.45 GHz.
 4. The method of claim 1 wherein said depositionis carried out under the existence of a magnetic field of 1 Kilo Gaussor stronger.
 5. The method of claim 1 wherein said productive gas isexcited in a mixed cyclotron resonance.
 6. The method of claim 5 whereinthe pressure in said reaction chamber is chosen between 0.1 Torr and 300Torr.
 7. The method of claim 6 wherein said nitrogen compound isammonia.
 8. The method of claim 6 wherein said productive gas includesat least one hydrocarbon.
 9. The method of claim 6 wherein saidproductive gas is C₂ H₆, C₂ H₄ and/or C₂ H₂.
 10. The method of claim 1wherein said productive gas is CH₄.
 11. The method of claim 1 whereinsaid carbon layer is a crystalline layer.
 12. The method of claim 1wherein said carbon layer is a micro-crystalline layer.
 13. The methodof claim 1 wherein said carbon layer is an amorphous layer.
 14. Themethod of claim 1 wherein said carbon layer is deposited repeatedlyfollowing or followed by the deposition or BN film in order toconstitute a super lattice structure.